A metal-air cell is an electrochemical cell that uses an anode made of a metal anode and oxygen from the air as a cathode depolarizer (fuel). Since oxygen is not stored in the cell, the capacity and energy density of metal-air cells can be very high. Specific energy for a zinc-air system and other metal air battery systems are shown in Table 1. The specific energy of zinc-air cells surpasses Li ion by a wide margin.
TABLE 1Theoretical specificTheoretical specificenergy, Wh/kgenergy, Wh/kgOpen-circuitBattery type(including oxygen)(excluding oxygen)voltage, VAluminum-air430081401.2Calcium-air299041803.12Magnesium-air278964622.93Potassium-air 93517002.48Sodium-air167722602.3Lithium-air5210111402.91Zinc-air109013501.65Lithium-ionN/A2653.8battery
During discharge, the anode in a metal-air cell undergoes oxidation reactions typical for the particular metal. The reaction mechanism is usually multi-step, with intermediates depending on the electrolyte. Typical anode discharge reactions for alkaline zinc-air cell are represented by equations 1-4.Zn+4OH−→Zn(OH)42−+2e−  (1)Zn+2OH−→Zn(OH)2+2e−  (2)Zn+2OH−→ZnO+H2O+2e−  (3)Zn(OH)2ZnO+H2O  (4)
The critical component of all metal-air cells is the air electrode. Unlike traditional battery cathodes, the air electrode can function mostly as a catalytic matrix for oxygen reduction and also acts as cathode current collector, while oxygen is supplied from the air outside the battery which is usually atmospheric air. The cathode reaction sequence for alkaline aqueous electrolytes is described by the following equations:O2+2e−+H2O→O2H−+OH−  (5)O2H−→OH−+½O2  (6)
Since oxygen is supplied from the outside air, the positive electrode itself can be very thin as it is needed only to conduct electrons and catalyze the oxygen to hydroxyl reaction. A variety of carbons, potassium permanganate, manganese dioxide, cobalt tetrapyrazinoporphyrazine derivatives, perovskites, titanium doped manganese dioxide, cobalt oxides, graphene, Ketjen Black, doped Ketjen black, carbon nanotubes, carbon fiber, cobalt manganese oxides, electrically conductive polymers and intrinsically conductive polymers, combinations thereof and other compounds have been reported to catalyze chemical decomposition of peroxides via reaction (6).
Typical air electrode cathode structures usually contain multiple layers, including an air diffusing layer, a hydrophobic PTFE layer, a catalytic layer where carbon is mixed with a catalyst which is active to improve peroxide decomposition. The cathode is separated from the anode by a separator and barrier layer, wettable to electrolyte, and capable of preventing the metal anode from direct contact with the cathode.
Air holes, facing the cathode, are used to provide adequate air supply. Before a metal-air cell is needed for use it must be protected from the external environment. An adhesive and gas differentiating tab covering the vent holes is used to prevent most of oxygen from freely entering the cell. The tab is removed prior to use to enable the cathode to freely air up.
Zinc-air cells are one of the most commonly used metal-air systems. The cell demonstrates higher energy density compared to alkaline cells. Disadvantages of this system include relatively low rate capability and susceptibility to environment conditions. Zinc-air cells typically perform well on low continuous current drains, but can provide insufficient service on intermittent and high rate tests.
Zinc-air cell construction is typically a coin cell design, which includes a bottom can with air holes, a cathode structure, including hydrophobic and barrier layers, on top of it, followed by separator. As the air electrode is thin, most of the cell volume is taken by zinc anode. Larger cells, typically in prismatic format, require a complicated air delivery system to sustain adequate performance. Cylindrical (e.g. AA or LR6 sized) zinc-air designs usually result in cell cost prohibitively high for the alkaline category, and require complicated cathode designs and manufacturing processes, air delivery paths and the need for welding a cathode current collector to the positive terminal assembly.
Typical primary zinc, manganese dioxide (“Zn/MnO2”) or alkaline batteries can provide good performance on variety of tests, including intermittent, with low material and manufacturing costs. However, battery capacity and hence service life is limited by amount of electrochemically active ingredients such as MnO2 and zinc which can be packed into the cell.
In zinc-air and air assisted zinc-MnO2 batteries the anode material and associated electrochemical reactions are essentially the same as for Zn/MnO2. However, in air assisted batteries, also known as air recovery or air restored batteries, the cathode can be recharged or restored by air. When the cell is not in use, or when the discharge rate is sufficiently low, atmospheric oxygen can enter the cell through the air holes and reacts with the air cathode, recharging manganese oxyhydrate back to dioxide:½O2+MnOOH→MnO2+OH−  (7)
At high discharge rates, an air assisted battery can operate like a conventional alkaline cell reducing fresh manganese dioxide. The cathode can be wetted with electrolyte for the MnO2 reduction to occur. At the same time a typical cathode must enable oxygen ingress as oxygen has low solubility in KOH solutions.
Air assisted batteries are particularly useful in intermittent use applications and can provide substantial performance improvement compared to traditional alkaline batteries. However, design of the air assisted batteries is as complicated as that of zinc-air cells, making them difficult to manufacture and costly. Because of these limitations, no practical cylindrically shaped air assisted battery has been mass produced to date. Therefore, the need for low cost and easy to manufacture battery with improved run time in consumer applications is still unfulfilled, and there exists a need for a metal-air battery that can operate in intermittent and high rate use applications.